1. Field of the Invention
The present invention relates to a bicycle sprocket, and more particularly to a sprocket for gearshift.
2. Description of the Related Art
FIG. 6 of the drawings illustrates a typical sprocket 20 having a number of annularly equidistant teeth 21, 22, 23. Two guiding surfaces 24 are diametrically provided on an outer side of the sprocket 20 and each extends over several teeth 22. Each guiding surface 24 has an end merges with a so-called easy-to-engage tooth 21. FIG. 7 shows a top view of the easy-to-engage tooth 21 that has a substantially rectangular crest 210. One of the teeth 22 covered by the guiding surface 24 and adjacent to the tooth 21 has a height identical to the remaining teeth 22. As a result, during gearshift from one sprocket to another, engagement of a chain with the sprocket 20 is obstructed by said one tooth 22 adjacent to the tooth 21. In order to solve this problem, an inner right section 211 of the crest 210 is beveled to form a quadrilateral. Thus, the right side of the tooth 21 is made thinner to allow smooth engagement of the chain onto the sprocket 20.
However, the beveling process may damage the inner right section 211 of the tooth 21 that is originally very thin. In addition, the left side of the easy-to-engage tooth 21 is not cut properly such that if an outer link plate of a chain is used to proceed with gearshift, an inner link plate on the right of the outer link plate has not moved across the tooth 21 when the outer link plate bears against a concave surface 212 on the right side of the tooth 21. As a result, the inner link plate of the chain will impinge on the left section of the tooth 21 when the inner link plate moves across the tooth 21. Such movement is not smooth and noise is generated. Furthermore, the tooth 22 located on the right side of the tooth 21 is not properly cut. When the inner link plate of a chain is used to proceed with gear shift, an end of the outer link plate on the left of the inner link plate bears against the concave surface on the right side of the tooth 21. In this case, the outer link plate of the chain has not moved across the tooth 21 As a result, the outer link plate of the chain will impinge on the tooth on the right side of the tooth 21 when the outer link plate moves across the tooth 21. Again, such movement is not smooth and noise is generated. Moreover, each tooth 23 has a beveled surface 230 that extends downwardly from the crest of the tooth 23. The portion below the beveled surface 230 has a thickness the same as that of the sprocket 20. However, the chain is slightly twisted during the gearshift. Two adjacent sprockets have a small distance therebetween, such that the chain will contact with the second tooth to the fourth tooth (depending on the number of the teeth of the sprocket) counting from the tooth 21 of the sprocket to be engaged. Again, such movement is not smooth and noise is generated.
The present invention is intended to provide an improved sprocket that mitigates and/or obviates the above problems.